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In three U.S. cities, Google cars roamed over the past year equipped with sensors to record methane leaks from utility pipes.

The Environmental Defense Fund, a New York nonprofit that advocates for environmental issues, partnered with the search engine giant to map these non-hazardous leaks that contribute to global warming.

The effort is one of 16 studies that EDF has undertaken to trace methane emissions in the natural gas production cycle, from the wellhead to homes.

Last year, the organization published the first in the series, dealing with emissions from shale gas wells. Based on a sample of 190 sites, EDF and the University of Texas concluded the annual leak rate from drilling and fracking activities is about 0.42 percent of gas produced.

The leak rates for distribution lines, which deliver natural gas to customers‘‍ homes, tend to be higher.

In interactive maps posted on EDF‘‍s website, www.edf.org/​climate/​methanemaps, the group showed that gas leaks in Boston and Staten Island were spotted at a rate of one per mile. In Indianapolis, the rate was one in 200 miles. Pittsburgh was not part of the study.

The difference between the three cities sampled likely lies in the infrastructure.

Boston and Staten Island have old pipelines, many made from cast iron and unprotected steel. Indianapolis pipelines are more modern and mostly made of plastic. Old and unprotected metal corrodes and leaks. New plastic doesn't.

That's why utilities in Pennsylvania, where nearly 12,000 miles of pipelines are of the old metal variety, are replacing their networks with plastic tubes. While historically the emphasis has been exclusively on public safety, questions of environmental consequence are also playing into the efforts now.

"I think there's generally an increasing consciousness about this," said Barry Kukovich, a spokesman for Peoples Natural Gas, the largest utility in southwestern Pennsylvania.

Gas utilities are required to calculate and report their lost gas rate to the state Public Utility Commission annually, but they don’‍t map the leaks and categorize them by flow rate the way EDF and Google did.

Last year, Peoples "lost" 4 percent of the gas that went into its pipelines, which is down from more than 6 percent a few years ago. The company plans to get that down to 3 percent by the end of this year, Mr. Kukovich said.

Equitable Gas, a subsidiary of Peoples, reported a loss rate of 2.5 percent in 2013, down from 4 percent in 2010.

Columbia Gas, which has been carrying out an aggressive pipeline replacement program over the past few years, decreased its lost gas rate to less than half a percent in 2013.

Losses don't all come from pipeline leaks. Some can be attributed to measurement errors, for example. Still, leaks constitute the vast majority of gas that never makes it to the customer — costing the utilities, their customers and the environment.

In 2012, the Public Utility Commission standardized the formula for calculating lost and unaccounted for gas across utilities in the state and set caps on how much of the cost customers can shoulder. By 2016, PUC rules say utilities cannot recover the cost for more than 5 percent of their lost gas from ratepayers. By 2020, the cap drops to 3 percent. Currently, there is no cap.

Methane isn't harmful to breathe and, when vented into the air, doesn't pose a safety hazard. But it does contribute to global warming since it's at least 21 times as potent at trapping heat in the atmosphere as carbon dioxide.

PUC regulators aren't necessarily focusing on curbing greenhouse gas emissions, but “a safe system that is operating with a focus on lost/​unaccounted for gas is a system that is a cleaner and more efficient,” said Jennifer Kocher, a spokeswoman for the agency. “The end result for the environment is the same no matter how we got there.”

The EDF study looked at so-called Class 3 leaks, which don't pose a safety hazard to people and property and aren't expected to do so anytime soon. Class 1 leaks are those that pose an immediate danger and must be repaired at once. Class 2 are not an immediate safety hazard but are typically taken care of within a year, according to Sue Fleck, vice president of the National Grid, which supplies gas to Boston and Staten Island customers.

Utilities tend not to fix Class 3 leaks specifically, Mr. Fleck said. Instead, those are addressed when pipes are replaced as part of longer-term infrastructure improvement programs.

Mapping the intensity of individual leaks, however, will allow National Grid to prioritize which pipelines it replaces first, thereby getting the most environmental bang for the buck, EDF's chief scientist Steve Hamburg said.

EDF and Google plan to send out the car-mounted sensors to more U.S. cities — and they're taking requests.

Mr. Kukovich said Peoples hasn't been approached but he praised the technology as "a great tool for the industry."

"We applaud it," he said. "Right now, we walk our lines with gas detection devices. I think this new tool is great."

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